In recent years, reproduction signal processing apparatuses such as DVD drives demand high-speed reading, and the speed of the reading has been becoming higher year after year. However, a higher reproduction speed leads to an increase in the frequency of an operation clock, which increases power consumption of an LSI that is used for internal signal processing or the like, resulting in a problem such as heating.
To suppress the power consumption, conventional reproduction signal processing apparatuses utilize a technology called half rate mode, which divides the frequency of a reproduction clock that is outputted from an oscillator so as to be in phase with clock components included in the reproduction signal, and uses the frequency-divided reproduction clock as an operation clock, thereby reducing the power consumption (for example, refer to a pamphlet of International publication No. 01/018809).
In the half rate mode, sampling points are obtained only by dividing sampling points which are obtained at the normal reproduction, i.e., at a time when a reproduction clock that is included in a reproduction signal is used as an operation clock (hereinafter, this is referred to as channel rate mode). Thus, the half rate mode obtains sampling points as many as those in the channel rate mode by interpolating sampling points that should have essentially been obtained by linear interpolation or the like. This interpolation enables to perform adaptive equalization control to the reproduction signal also in the half rate mode, with a circuit structure that is almost the same as in the channel rate mode.
However, when a waveform including a large nonlinear distortion that is called asymmetry is inputted to the conventional reproduction signal processing apparatus, an erroneous equalization target value is set in a provisional judgment means which is used for adaptive equalization control both in the channel rate mode and in the half rate mode, whereby an equalization error is increased and then the adaptive equalization control becomes unstable.
Further, when a waveform having a distortion that is generated in playing a disc of bad pit shape at the high-speed recording is inputted to the apparatus, the equalization error becomes large both in the channel rate mode and in the half rate mode, whereby the adaptive equalization control becomes unstable.